SWEETAPPLE: POSSUM DIET IN BEECH FOREST 157 Possum (Trichosurus vulpecula) diet in a mast and non-mast seed year in a New Zealand Nothofagus forest P. J. Sweetapple Landcare Research, P.O. Box 69, Lincoln 8152, New Zealand (E-mail: [email protected]) __________________________________________________________________________________________________________________________________ Abstract: The annual diet of possums (Trichosurus vulpecula) during both a beech (Nothofagus) mast fruiting year and a non-mast year in the simple beech forests of the North Branch of the Hurunui Catchment, eastern South Island, New Zealand, was determined by sorting the contents of 270 possum stomachs, collected between December 1999 and December 2001. Beech flowers and seeds contributed 46.1% to annual diet during the mast year, but were not eaten during the non-mast year. Beech foliage and bark made up 13.2% and 45.0% of annual diet in the mast and non-mast years, respectively. Fungi, herbs and grasses together comprised 23.1% and 42.4% of the annual diet in the mast and non-mast years, respectively. Diet varied with altitude and distance down the valley, principally in the relative contributions of foods from the three beech species present, which reflected local canopy composition. Other local food sources such as valley-floor herbs and grasses also contributed significantly to local diet. Although heavily reliant on beech species, possums are unlikely to have a significant impact on beech flower and seed production, or on the health of beech trees because of possums’ low abundance and the large beech biomass at this site. Recognised possum-preferred foods such as Aristotelia serrata, Fuchsia excorticata, Hoheria glabrata, Raukaua simplex, Elaeocarpus hookerianus and mistletoes (Peraxilla tetrapetela and Alepis flavida) were all strongly preferred by possums but, because of their scarcity in the study area, were only eaten in small quantities. __________________________________________________________________________________________________________________________________ Keywords: diet; mast fruiting; possum; simple Nothofagus forest; Trichosurus vulpecula. Introduction threaten indigenous forest ecosystems through their herbivory and predation [see Payton (2000) and Sadleir Beech (Nothofagus) species exhibit mast seeding with (2000) for reviews]. In many habitats possums eat large quantities of seed (> 4000.m-2) produced at large quantities of fruit and seed when these are intervals averaging c. 5 years, with little or no seed available (Nugent et al., 2000). Periods of heavy fruit produced in many of the intervening years (Wardle, utilisation have been linked with enhanced breeding 1984). Partial mast years (500–4000 seeds.m-2) are success in possums (Bell, 1981), and possums have sometimes observed between full mast years (Wardle, been shown to suppress fruit production in some tree 1984). Beech masting is a significant ecological event species (Cowan, 1990a; Cowan and Waddington, in beech forest: heavy flower and seed production 1990). Beech masting may, therefore, affect possum triggers eruptions in forest-litter arthropod populations diet and fecundity, and possum feeding may affect (Alley et al., 2001) and increased breeding activity in beech seed production, but these potential interactions some indigenous birds such yellow-crowned parakeet have not been quantified. Furthermore, there are no (Cyanoramphus auriceps) and kaka (Nestor published data on possum diet from the floristically meridionalis) (Elliott et al., 1996; Wilson et al., 1988). simple beech forests that dominate the indigenous Beech mast events also trigger perturbations in the forests east of the Main Divide of the Southern Alps in densities of introduced mammalian pests, including the South Island and the axial ranges of the North house mice (Mus musculus) (King, 1982, 1983; Island (Wardle, 1984). Possum diet has been described Murphy, 1992; Fitzgerald et al., 1996; Alley et al., from some western South Island beech forests (Owen 2001) and stoats (Mustela erminea) (King, 1983). and Norton, 1985; Cochrane et al., 2003). Elevated stoat densities often result in increased The New Zealand Department of Conservation predation on indigenous birds (Murphy and Dowding, (DOC), Canterbury Conservancy, contracted Landcare 1995; Elliott, 1996; Elliott et al., 1996; O’Donnell and Research, Lincoln, New Zealand, to determine possum Phillipson, 1996; Wilson et al., 1998). diet from stomachs collected from the North Branch Possums (Trichosurus vulpecula) are one of New Hurunui Catchment, eastern South Island in 1999– Zealand’s most pervasive mammalian pests, and 2001. Seedfall trays in the adjacent South Branch New Zealand Journal of Ecology (2003) 27(2): 157-167 ©New Zealand Ecological Society 158 NEW ZEALAND JOURNAL OF ECOLOGY, VOL. 27, NO. 2, 2003 Hurunui Catchment caught 160.7 kg.ha-1 of beech throughout, but dominating the canopy at high altitudes seed from March to May 2000 (A. Grant, Department and in wetter western parts of the study area. Mountain of Conservation, Christchurch, N.Z., unpubl. data). beech (N. solandri var. cliffortioides) is common in the This equates to c. 4000 seeds.m-2 (Wardle, 1984: p. drier eastern parts of the study area, particularly at high 256) indicating that the first year of the study was a full altitudes. In total, beech species comprise 82% of the beech-mast year. This estimate is likely to underestimate estimated total plant biomass in the study area: 21%, total seedfall as small quantities of seed continue to fall 27% and 34% for mountain, red and silver beech, during winter and, for silver beech, spring (Wardle, respectively [data derived from the National Vegetation 1984). The following year, 2001, was a non-mast year Survey (NVS) databank]. The sparse non-beech with just 0.5 kg.ha-1 of beech seedfall in March to May. components of these forests are dominated by small- The diet study provided the opportunity to compare leaved Coprosma species, Griselinia littoralis, possum diet in a beech mast and non-mast year, as well Phyllocladus alpinus and Myrsine divaricata. Possum- as to document possum diet from a simple eastern preferred tree species are uncommon or rare, ranging beech forest and assess variation in diet with altitude, from about 0.5% [Raukaua simplex (Mitchell et al., aspect and geographic location. The results of this 1997), Hoheria glabrata] to less than 0.05% (Aristotelia work are presented in this paper. serrata, Fuchsia excorticata) of estimated plant biomass (NVS databank). Subalpine and alpine shrublands, grasslands and herbfields are present above Methods the upper limit of forest. Possums have been present in the study area since Study area at least 1963 (Cowan, 1990b) and leg-hold trap-catch The study area consisted of the forested parts of the rates of 8–16 possums per 100 trap-nights during the upper North Branch Hurunui Catchment, North study period (A. Grant, Department of Conservation, Christchurch, N.Z., unpubl. data) indicate a low-density Canterbury, New Zealand, from Harper Pass on the -1 Main Divide of the Southern Alps to Mackenzie Stream, possum population (c.1–2 possums.ha : B. Warburton, 14 km east of the Main Divide (Fig. 1). The valley floor Landcare Research, Lincoln, N.Z., pers. comm.). is characterised by broad river flats at 600–700 m a.s.l., much of which are open grasslands dominated by Possum stomach collection exotic species. The valley sides are almost entirely Stomachs were taken from possums caught on the forested by beech up to c. 1300 m a.s.l. Red beech same six transects during each sampling trip. Transects (Nothofagus fusca) occurs on the lower and mid- were subjectively located to sample the north and altitudes slopes, with silver beech (N. menziesii) present south sides of the valley, and the western, middle and River/streams N Forest margins Mackenzie Stream Sampling transects C Cyanide-poisoned (May) T Trapped (March) 0 1 2 C km Lake Sumner 5 km Landslip Stream Blue Stream T C T T T T T C C a T m e C r T o C n T S t r e a C m Harper Pass Figure 1. The North Branch Hurunui study Upper Valley Mid-Valley Lower Valley Transects Transects Transects area and location of sampling transects. SWEETAPPLE: POSSUM DIET IN BEECH FOREST 159 eastern parts of the study area (Fig. 1). They ran from The percent dry weight for each food item identified the forest-grassland margin on the valley floor to the was calculated for each stomach. The mean percent upper altitudinal limit of tall forest. Between 13 and 25 dry weight (all stomachs) for each collection was then sampling stations were established at approximately calculated for each food item, and the annual means 25-m vertical intervals along each transect. Cyanide were calculated by averaging the collection means. baits (Feratox®, Feral Control, Auckland) were laid in Differences between years in the contribution to diet of bait stations at each sampling station for about 4 days food items that comprised more than 0.5% of annual (range: 2–5 days), in December, February/March, May/ diet in either year were tested using Kruskal-Wallis June, and September, in each of the two years. A total tests. Because the sampling regime was balanced (strata of 360 possum stomachs were collected during the were sampled in proportion to their size, with the same study. Of these, 270 were sorted and analysed (30 in lines sampled each season, and the same seasons February/March in both years and 35 for all other sampled each year) there was no need